When certain metals or metal alloys are remelted or held in molten state in a furnace or crucible, the metals partially oxidize to form a dross comprising primarily the oxide of that metal and the metal itself, together with smaller quantities of impurities and alloying elements present in the molten metal and/or compounds (such as oxides and halides) of these impurities and alloying elements. The dross floats on the surface of the molten melt body and is separable therefrom by skimming or other techniques. It is usually regarded as a waste product in the metals industry, which has the problem of disposing of copious amounts of the dross, although procedures are known for recovering metal or other values from dross. For example, procedures are known for obtaining usable activated alumina from dross (as disclosed in U.S. Pat. No. 4,075,284), and for converting dross to a usable refractory material by compacting and firing in a kiln (as disclosed in U.S. Pat. No. 4,523,949).
As used herein, the term "dross" means a solid phase material, usually oxide- or nitride-rich, which has formed on the surface of a body of molten metal, or at the three phase junction between the furnace wall, molten metal and gas atmosphere, during a metal melting operation. It is a physical mixture of entrapped metal and a ceramic formed by oxidation. The dross is a thin, weak, floating layer which typically is removed as a waste material by physical separation from the molten metal body, for example by skimming operations.
Dross skimmed from the surface of molten metal may contain as much as 60% by weight metal, some of which forms as large, shapeless inclusions. The dross is usually passed through a crushing operation, and the larger metal pieces are physically separated from the friable ceramic, and the removed metal is returned to the melting furnace. Another source of dross is foundry operations such as the chlorination of reactive metal components of the melt or the addition of foundry fluxes. Dross generated in such operations contains soluble salts which would appear to hamper other uses.
In recent years there has been an increasing interest in substituting ceramics for metals because, with respect to certain properties, ceramics are superior to metals. There are, however, several known limitations or difficulties in making this substitution such as scaling versatility, capability to produce complex shapes, satisfying the properties required for the end-use application, and costs. Many of these limitations or difficulties have been overcome by the inventions disclosed in patent applications assigned to the same assignee as this application and discussed in the subsequent section, which provide novel methods for reliably producing ceramic materials, including shaped composites. A compatible filler for the composite is required for ceramic composites in order to attain the desired end properties.